Method of electrodepositing chromium directly on aluminum



Patented Dec. 8, 1953 METHOD onnnnornbnnrosrfme' CHRO- IWIU-MDIRE OTLY ON ALUMINUM Frank Bassal,- 'Detroit, Mich.-, assig'nor' toUnited Chromium,-=Incorporated, New York, N. Y., a corporation of Delaware-- No Drawing... Application September 8, 1950,"

. Serial No. 183,905

3 Claims; (01. 204-33) 1 r p, -This-=invention "relatesto electrodepositlng chromium directly on aluminum-and provides improvements therein.

The-present invention provides a method of electrodepositing chromium directlyf'on aluminum with good adherenceand without limit as'to thickness, particularly as afiecting the adherence of*the-chromium"-to the aluminum.

7 The procedure which is followed in carrying outsaid "method is as follows:

(1) An aluminum article on whichchromium is to be electrodeposited" is immersed for about 10 to l secondsinan aqueous alkaline zincate cupro-cyanide solution from which "an -immer-' sion-coatingof zinc and'copper is deposited on the aluminum: The preferred composition of-the dip solution-forproviding the immersion coating of -zincand copper is asfollowsz'" Bath solution Zinc-cinder; 90g./l. (12'ozs/gal.) Sodium hydroxide; "429 g./l. (60 ozs/gal.) Copper: cyanide; .7.5 g./'l. '(1 "oz. gal.) sodium-cyanidezin; 15 g./l. (2 ozs/galz') T01 prepare and. replenish the above .dip solution a solid composition of matter may be .used containing :by weight:.-

- Per cent Zinc-oxide e. 16.6 Sodium hydroxide L; .793 Copper-cyanide 1.4 Sodiummyanide 2.7

The 'alkalr' metal I zincate solution" of" Hewitsdn Patent-'No; 1,627.900 is nota substitute for the alkaline zincate cuprocyanidesolutions (2) --=The zinc and copper "-immersion' coating on t-he aluminum'article is -rem'oved; a concen trated aqueous solution "of nitric-acidjin which the coated-aluminum article is immersed, being used for this purpose. The. article is immersed forlflto IQSlsecOnds, with the nitric acid solution at room temperature'this being suflicient time forthe removal of the zinc and copp er'ime' mersionacoatings I -'(3f) ,Thesaluminum..articleiis :immersedkior fa second time in the aqueous alkaline zincate cuprocyanide solution referred to above, the time of .immersion being usually between and seconds. out of the bath has another zinc and copper immersion coating thereon. The second immersion in the alkaline zincate cuprocyanide dip is used since the first zinc and copper immersion coating has a tendency to be non-uniform and does .1

The aluminum article when it comes r 2;: not :dissolve instantlyon immersion in a chromium .platingsba'thr Unless the second dipis used, the adhesion of. the chromium electrodeposit will be' more critical and less dependable-.-=

4) v.The' aluminum article with the zinc 'and copper immersion coating thereon is then immersed. in .a knownchromic acid-catalyst acid radical chromium plating bath. v The zinc'and copper immersion coating on the aluminumarti cle being extremelythin, will dissolve almost'ine stantly following immersion in a standard-chromiumplating-bath, at both room andelevated temperatures. The aluminum article is connected inanelectric circuit as a cathode-, butthe cir cuit connectionneed not-necessarily be made at thetime the zinc and copper coated aluminum article is first immersed in the solution. However,.thecircuitshould be completed so that electric current-passes to the aluminum article tfrom which the zinciand .copperimmersion coating has been removed): within 15 seconds after the article has-been immersed in the chromium plating electrolyte. The plating of the aluminum can be conducted as long as may benecessaryto obtain a desired thickness of chromium. C'hromium deposits of several thousandths of-an-inchhave been obtained without any efiecton the good ad= herence .of-the; chromium to the aluminum being indicated.

- In accordance with-usual good practice-in electroplating, the aluminumarticle is rinsed in waterafter removal from each dip-solution and bath, andmoreover.. and also in accordancewith good practice. in electroplating, the aluminum articleis givenapreliminary cleaning. Also it is the usual-practice inplating on aluminum to etch the surface.

A suitable preliminary cleaningand etching procedure is as follows:

1 Thefinished aluminum-article is first'degreased bydipping in I a grease solvent such as trichlor ethylene. *Thenit is dipped in an alkalinesolution, which solution may be a g./l.-sodium*h-y= droxide' solution having a temperature of about The akaline 'solution slightly etchesthe aluminum. Upon removal- F from" the alkaline solution "the aluminumflarticlel is rinsedwith water and thereafter dipped in an acid pickle for about 1 to 5 minutes. The acid pickle may consist of CrO335 g./l.; and H2SO41'73 g./l., and have a temperature of 150 F. After removal from the acid pickle the aluminum article is again given a water rinse, and is then dipped in a concentrated nitric acid (aqueous) solution for 5 to 10 seconds at room temperature. Following Nominal composition of wrought aluminum alloys Percent of Alloying ElementsAluminun1 and Normal Impurities Oonstitute Remainder Copper Silicon Manganese Magnesium Chromium In general the foregoing procedure has been found to give satisfactory results with aluminum alloys having a magnesium content less than about 2.5 per cent and silicon content less than 2 per cent. For higher silicon content alloys and for aluminum castings containing less than about 2.5 per cent magnesium, the following modified procedure can be followed. In the preliminary cleaning, before the immersion of the aluminum article in the acid pickle, the aluminum article is given an additional dip in a solution of nitric and hydrofluoric acids, such as one containing 3 parts concentrated nitric acid (70% by weight) to 1 part of concentrated hydrofluoric acid (29% by weight) for abouts to 5 seconds. Alternatively, after the removal of the aluminum article from the acid pickle it may be scrubbed with a tampico brush to remove loosely adherent smut.

The time of the immersion of the aluminum article in the alkaline zincate cuprocyanide solution is a matter of importance, and for aluminum and the alloys listed above the time ofimmersion should not exceed the given values. It is possible that other less common aluminum alloys may require more or less time of immersion in the alkaline zincate cuprocyanide solution. The exact time limits for any particular alloys can be determined by making a preliminary test.

The present method of electroplating chromium on aluminum has results which differ from prior practice in two important particulars. The chromium is directly bonded to the aluminum, and any thickness of chromium desired can be electroplated directly on the aluminum. Chromium thicknesses from to mills have been obtained without any spalling, blistering or any other indication of poor or faulty adhesion.

What is claimed is:

1. A method of electrodepositing chromium directly on aluminum, consisting essentially of immersing an aluminum article to be electroplated for about 10 to 15 seconds in an aqueous alkaline zincate cuprocyanide solution to form a zinccopper immersion coating thereon, removing said article and immersing the same in a nitric acid solution to remove said zinc-copper immersion coating, immersing the article a second time for not more than 15 seconds in the aqueous alkaline zincate cuprocyanide solution to form another zinc-copper immersion coating thereon, removing the article from the latter solution, then immersing said coated article in a chromic acid chromium plating bath to completely strip the zinc-copper immersion coating and within 15 seconds passing electric current to said immersed stripped aluminum article as a cathode to electrodeposit chromium thereon, thus producing a chromium plated aluminum article having no intermediate coating between the chromium and the aluminum.

2. A method of electrodepositing chromium directly on an aluminum-containing article selected from the class consisting of articles of aluminum and high aluminum alloys, consisting essentially of immersing the article to be electroplated in an aqueous alkaline zincate cuprocyanide solution to form a zinc-copper immersion coating thereon, removing said article from the solution and dissolving said zinc-copper immersion coating therefrom, immersing the article a second time in the aqueous alkaline zincate cuprocyanide solution to form another zinc-copper immersion coating thereon, removing the article from the latter solution, then immersing said article in a chromic acid chromium plating bath to completely remove the zinc-copper immersion coating thereon by the action of the chromium plating solution, and, immediately after the removal of said coating, electrodepositing chromium thereon, thus producing a chromium plated article having no intermediate coating between the chromium and the article.

3. A method of electrodepositing chromium directly on an aluminum-containing article selected from the class consisting of articles of aluminum and high aluminum alloys, and wherein the thickness of the chromium deposit ranges from a fraction of a mil to at least 10 mils, which comprises immersing an article to be electroplated in an aqueous alkaline zincate cuprocyanide solution for a time just sufiicient to form a zinc-copper immersion coating thereon, removing said article from the solution and dissolving said zinc-copper immersion coating therefrom, immersing the article a second time in the aqueous alkaline zincate cuprocyanide solution for a time just sufficient to form another zinc-copper immersion coating thereon, removing the article from the latter solution, immersing the coated article in a chromic acid chromium plating bath to completely remove the zinc-copper immersion coating by the action of the chromium plating solution, and, immediately after the removal of said coating, electrodepositing chromium on the article, thereby producing a chromium plated article having no intermediate-coating between the chromium and the article.

FRANK PASSAL.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,091,386 Viers Aug. 31, 1937 2,142,564 Korpiun Jan. 3, 1939 

1. A METHOD OF ELECTRODEPOSITING CHROMIUM DIRECTLY ON ALUMINUM, CONSISTING ESSENTIALLY OF IMMERSING AN ALUMINUM ARTICLE TO BE ELECTROPLATED FOR ABOUT 10 TO 15 SECONDS IN AN AQUEOUS ALKALINE ZINCATE CUPROCYANIDE SOLUTION TO FORM A ZINCCOPPER IMMERSION COATING THEREON, REMOVING SAID ARTICLE AND IMMERSING THE SAME IN A NITRIC ACID SOLUTION TO REMOVE SAID ZINC-COPPER IMMERSION COATING, IMMERSING THE ARTICLE A SECOND TIME FOR NOT MORE THAN 15 SECONDS IN THE AQUEOUS ALKALINE ZINCATE CUPROCYANIDE SOLUTION TO FORM ANOTHER ZINC-COPPER IMMERSION COATING THEREON, REMOVING THE ARTICLE FROM THE LATTER SOLUTION, THEN IMMERSING SAID COATED ARTICLE IN A CHROMIC ACID CHROMIUM PLATING BATH TO COMPLETELY STRIP THE ZINC-COPPER IMMERSION COATING AND WITHIN 15 SECONDS PASSING ELECTRIC CURRENT TO SAID IMMERSED STRIPPED ALUMINUM ARTICLE AS A CATHODE TO ELECTRODEPOSIT CHROMIUM THEREON, THUS PRODUCING A CHROMIUM PLATED ALUMINUM ARTICLE HAVING NO INTERMEDIATE COATING BETWEEN THE CHROMIUM AND THE ALUMINUM. 